In organic light-emitting diodes (OLEDs), only part of the generated light is outcoupled directly. The rest of the light generated in the active region is distributed among various loss channels, for instance in light which is guided in the substrate, in a transparent electrode and in organic layers by wave guidance effects, and in surface plasmons, which may be generated in a metallic electrode. The wave guidance effects arise in particular through the refractive index differences at the interfaces between the individual layers and regions of an OLED. With known OLEDs, typically only around a quarter of the light generated in the active region is outcoupled into the surrounding environment, i.e., for example, air while around 25% of the generated light is lost to emission by wave guidance in the substrate, around 20% of the generated light by wave guidance in a transparent electrode and the organic layers and around 30% through the generation of surface plasmons in a metallic electrode.
The organic semiconductor materials used in the organic light-emitting diodes have a refractive index in the range of for instance n˜1.8. Because of that relatively high refractive index, a major part of the light generated does not leave the organic layer stack due to total reflection at the interfaces with material layers with a lower refractive index (for example, the substrate glass with n˜1.5 or air with n˜1), thereby impairing the efficiency of the devices.
Methods of providing organic semiconductor materials with a reduced refractive index are known. It is known, for example, to modify the chemical structure of individual organic semiconductor materials such that the refractive index is decreased, or to use specific individual materials with a low refractive index. However, the known measures are limited to the use of those materials in an individual charge transport layer in the organic layer stack. Moreover, there are limits to reduction of the refractive index of the organic semiconducting materials by chemical modification.
It could therefore be helpful to provide an organic light-emitting device with efficient light outcoupling. In particular, it could be helpful to appropriately decrease the refractive index within the organic layer sequence.